This invention relates to the damping of vibrations in rotating devices. The invention has particular application to camshaft and sprocket assemblies for internal combustion engines.
Engine timing systems typically include an endless chain that drives between a driving sprocket on an engine crankshaft and a driven sprocket on an engine camshaft. The camshaft and sprockets may undergo resonance at certain frequencies. Vibrations from the resonance are often transferred through the system and can significantly increase the load on the system and components, possibly even causing chain breakage. This problem is particularly acute in engine systems with overhead camshafts, because the distance between the sprockets, and hence the length of the chain, is typically substantial and vibration effects are thereby magnified.
Conventional approaches to this problem have focused on reducing rotational perturbation of the crankshaft, by means of internal devices such as counter-rotating balance shafts, Lanchester dampers, and harmonic balancers. External devices such as fluid engine mounts and engine mounts having adjustable damping characteristics have been used. By contrast, the present invention focuses on reducing torsional vibrations of the camshaft.
Some prior art timing systems use a rubber damper piece placed against the sprocket and bolted to the camshaft to absorb vibrations. However, the rubber damper piece tends to fracture from the vibrations. Other timing systems employ a weight that, rather than being bolted to the camshaft, is positioned on the camshaft and held against the sprocket by a Belleville washer. Frictional material is placed at the area of contact between the sprocket and the weight. These systems, while effective, have drawbacks in terms of production, assembly, and durability.
An example of prior damping techniques is found in Wojcikowski, U.S. Pat. No. 4,317,388 issued Mar. 2, 1982. That patent discloses a gear with split damping rings of a diameter slightly smaller than the gear bolted to each side of the gear with a tapered bolt and nut assembly. Tightening of the bolts cams the damping rings outward, producing pressure circumferentially against the rim of the gear and causing tensile stresses on the gear. Additionally, tightening the bolts presses the elastomeric washers associated with the bolt and nut assembly firmly against the web of the gear, which damps the stress wave passing from the rim through the web and into the shaft. In contrast to this prior art structure, the present invention retains the damping piece in place with a hub member or retaining ring, thus obviating the need for bolts and nuts which may loosen. Further, the present invention does not require the precision forming of tapered bolt holes in the damping piece.
Another example of known damping techniques is Funahashi, U.S. Pat. No. 5,308,289 issued May 3, 1994. The damper pulley disclosed therein consists of a pulley joined to a damper-mass member with a resilient rubber member. The pulley and the damper-mass member each have at least two projections, and the projections of the pulley contact the sides of the projections of the damper-mass member. A second resilient rubber member is placed between the contacting projections. Bending vibrations from the crankshaft cause the pulley to vibrate in the radial direction and the first resilient rubber member deforms, causing the dynamic damper to resonate with the pulley and restrain the bending vibrations. Torsional vibrations cause the pulley to vibrate in the circumferential direction. The second resilient rubber member undergoes compression deformation, decreasing the spring force and raising the resonance frequency against the torsional vibrations. The present invention avoids the use of rubber, which has wear problems in use.
Another example of prior damping techniques is Kirschner, U.S. Pat. No. 4,254,98 issued Mar. 10, 1981. That patent discloses a damping ring for rotating wheels that includes a viscoelastic damping material disposed within an annular groove in the surface of the wheel. A metal ring is positioned in the groove on top of the damping material. In operation, the damping material undergoes shear deformation. The invention of Kirschner is particularly applicable to railroad wheels and the attenuation of screeching noise therefrom.